Wind Dispersal of Artifical Fruits Varying in Mass, Area, and Morphology

We used a field experiment to quantify the independent effects of variation in several fruit characteristics and in wind speed on dispersal of the windblown samaras of the tropical tree, Tachigalia versicolor. On Barro Colorado Island, Panama, we simultaneously released 15 different types of artificial models of these fruits, varying in seed mass, fruit area, and basic morphology, from the top of a 40—m tower, dropping them under five different wind conditions. In general, the horizontal distance a model fruit moves from the base of the tower is proportional to the average wind speed to which it was subject during the whole of its descent, divided by the square root of its wing—loading (wing—loading is the force of gravity on the fruit divided by its projected surface area). This relation was observed for fruits with masses from 1.0 to 1.9 g, with areas from 9 to 76 cm 2 , and with (wing—loading) 1 / 2 from 15.3 to 30.6 mPa. Two separate changes in the design of the fruit, but with no change in the wing—loading value from that of T. versicolor, caused the models to have either a more rapid rate of descent despite the appropriate aerodynamic motion, or an improper aerodynamic motion resulting in a pronounced increase in rate of descent. Model fruits that had the lowest wing—loading (achieved by decreasing seed mass or increasing fruit area) or that were blown by the strongest mean winds were dispersed the greatest mean distances and were scattered most evenly and over the widest area. A change in wing—loading or wind speed changed dispersal distance the least and the evenness of the distribution the most. Variation in wind speed has the potential to override the effect of variation in mass or area on dispersal, unless the differences in mass or area among models are quite large.